Add metrics naming convention for prometheus

## Describe the proposal

[Prometheus naming](https://prometheus.io/docs/practices/naming/)

### Add prefixes (based on business scenarios)
For provider: `dubbo.provider`, such as `dubbo.provider.rt.qps.seconds`
For consumer: `dubbo.consumer`, such as `dubbo. consumer.rt.qps.seconds`
For registry: `dubbo.registry`, such as `dubbo.registry.requests.total`

### Add suffix

#### Step 1: add [base-units](https://prometheus.io/docs/practices/naming/#base-units)
- Time: suffix with `seconds`, such as `dubbo.rt.qps.seconds`
- Length: meters
- Bytes: bytes
...

#### Step 2: add suffix by metrics type
- Counter: suffix with `total`, such as `dubbo.rt.requests.total`

#### Step 3: add other suffix
- The num of metrics: `count`, such as `dubbo.rt.seconds.count`
- Summary value: `sum`, such as `dubbo.rt.seconds.sum`
- Minimum value: `min`, such as `dubbo.rt.seconds.min`
- Maximum value: `max`, such as `dubbo.rt.seconds.max`
- Average value: `avg`, such as `dubbo.rt.seconds.avg`
- P99 value: `avg`, such as `dubbo.rt.seconds.p99`
- P95 value: `avg`, such as `dubbo.rt.seconds.p95`

Start with the linked Prometheus naming guidance and inspect Dubbo's existing metrics implementation and exposed metric names. Clarify which files, metric families, and naming rules are in scope; done should mean the agreed provider, consumer, and registry names follow the documented prefix and suffix conventions.

Written by the indexing model from the issue text.